1. Field of the Invention
The present invention relates to improved generation of motion compensated images. In particular, the present invention relates to a method for generating motion compensated output image data based on an interlaced input video sequence of subsequent fields and a corresponding motion compensator.
2. Description of the Related Art
The present invention is employed in picture improvement algorithms which are used, in particular, in digital signal processing of modern television receivers. Specifically, modern television receivers perform a frame-rate conversion, especially in a form of an up-conversion using frame repetition or a motion compensated up-conversion, for increasing the picture quality of the reproduced images. Motion compensated up-conversion is performed, for instance, for video sequences having a field or frame frequency of 50 Hz to higher frequencies like 60 Hz, 66.67 Hz, 75 Hz, 100 Hz, etc. While the 50 Hz input signal frequency mainly applies to a television signal broadcast based on the PAL or SECAM standard, NTSC based video signals have an input frequency of 60 Hz. A 60 Hz input video signal may be up-converted to higher frequencies like 72 Hz, 80 Hz, 90 Hz, etc.
During up-conversion, intermediate images are to be generated which reflect the video content at positions in time which are not represented by the 50 Hz or 60 Hz input video sequence. For this purpose, the motion of objects has to be taken into account in order to appropriately reflect the changes between subsequent images caused by the motion of objects. The motion of objects is calculated on a block basis, and motion compensation is performed based on the relative temporal position of the newly generated image between the previous and subsequent image.
For a motion vector determination and compensation, each image is divided into a plurality of blocks. Each block is subjected to motion estimation in order to detect a shift of an object from the previous image.
In contrast to interlaced video signals like PAL or NTSC signals, motion picture data is composed of complete frames. The most widespread frame rate of motion picture data is 24 Hz (24p). When converting motion picture data for display on a television receiver (this conversion is called telecine), the 24 Hz frame rate is converted into an interlaced video sequence by employing a “pull down” technique.
For converting motion picture film into an interlaced signal conforming to the PAL standard, having a field rate of 50 Hz (50i), a 2-2 pull down technique is employed. The 2-2 pull down technique generates two fields out of each film frame, while the motion picture film is played at 25 frames per second (25p). Consequently, two succeeding fields contain information originating from the same frame and representing the identical temporal position of the video content, in particular of moving objects.
When converting motion picture film into an interlaced signal conforming to the NTSC standard, having a field rate of 60 Hz (60i), the frame rate of 24 Hz is converted into a 60 Hz field rate employing a 3-2 pull down technique. This 3-2 pull down technique generates two video fields from a given motion picture frame and three video fields from the next motion picture frame.
The telecine conversion process for generating interlaced video sequences in accordance with different television standards is illustrated in FIG. 2. The employed pull down techniques result in video sequences which include pairs or triplets of adjacent fields reflecting an identical motion phase. A field difference for distinguishing a telecine signal from an interlaced image sequence, can only be calculated between fields which stem from different film frames.
For picture improvement processing, the temporal position reflected by each field in a sequence of interlaced video images does not need to be taken into account if the image content does not include moving objects. However, if moving objects are present in the fields to be processed, the individual motion phase of each field needs to be taken into account. Thus, picture improvement processing requires information indicating the motion characteristic of the individual fields, i.e. whether each field reflects an individual motion phase or whether a pull down technique has been employed, such that subsequent fields reflect identical motion phases.
In accordance with the different types of input image data, motion compensated de-interlacing processing is controlled in order to achieve the best picture quality of the output image data. For this purpose EP-A-0 740 467 and U.S. Pat. No. 6,222,589 describe a recombination of source fields which belong to the same film frame and a repeated output of the same frame with the desired output frame rate. In this manner, a high spatial resolution can be achieved. U.S. Pat. No. 5,495,300 describes an algorithm to double the motion phase from 25 to 50 per second in a 100 Hz television system. Motion vectors are estimated and employed between fields of different motion phases only. EP-A-0675 643 describes a two step picture improvement processing. In the first step, two fields of the same film frame are combined into an interlaced-to-progressive converter. The second step includes vector-based interpolation to generate motion compensated output images in between. For instance, a 50 Hz to 100 Hz conversion output sequence of four fields consists of an original input field and three interpolated output fields.
In order to accurately switch the motion compensation algorithms for film mode and video mode processing, a film mode detection is required. Film mode detection determines the current mode of the input image sequence based on global field differences. A known method is described in EP-A-1 198 137.
The present invention aims to further improve motion compensated image processing and to provide an improved method for generating motion compensated output image data and an improved motion compensator.
This is achieved by the features of the independent claims.